1. Field of the Invention
This invention relates to centrifugal pumps, and specifically relates to centrifugal pumps of the pitot tube type which are structured to provide operational and economical advantages over conventional high-speed centrifugal pumps.
2. Description of Related Art
Centrifugal pumps are used in a variety of industries to process fluids or liquid/solid mixtures. The speed at which a centrifugal pump operates varies widely between types and manufactures of pumps, and the speed at which any given pump is operated is dependent on the particular application in which the pump is used. Each application dictates the particular pumping requirements, and thus the particular characteristics (pump speed, pump size, cost to operate) that the pump must have to suit the given application.
For example, in certain applications it may be necessary or desirable to rapidly decrease the pump speed capacity of the pump from 100% to, say, 10% in a matter of a few seconds. Conversely, it may also be desirable to rapidly increase the pump speed capacity of the centrifugal pump from, say, zero percent to 80% or greater in a matter of a few seconds. In other applications, for example, the space available for placement of the pump at a job site may be very limited and will require a pump with a smaller footprint. And in most applications, the user of the pump is desirous of employing the most cost-effective pump system for the particular project.
These application requirements, as well as other requirements, have been addressed in the past by using conventional centrifugal pumps coupled with standard induction motors. Centrifugal pumps of the pitot tube type, however, have not been found economically and operationally suitable for some applications because pitot tube pumps have size, weight and inertia characteristics which rendered them less desirous, despite the fact that pitot tube pumps have pumping characteristics and efficiencies that make them highly suited to various industrial pumping applications.
By way of example, the rotational speed of conventional high-speed centrifugal pumps is better able to be rapidly increased or decreased because of the construction of such pumps. That is, the rotating assembly of a high-speed centrifugal pump comprises a small drive shaft and small impeller. Thus, the rotating mass and inertia of the rotating elements of a high-speed centrifugal pump are relatively low, and the speed of rotation of the elements can be rapidly increased or decreased as a result.
Centrifugal pumps of the pitot tube type, however, differ from conventional centrifugal pumps in that they have a rotor, driven by a drive shaft, which rotates about a stationery pitot tube. The pitot tube is positioned, and operates, to receive and move fluid from the pump at high velocity. Pitot tube pumps differ further from conventional centrifugal pumps in that the rotating assembly, comprising a rotor, rotor cover and drive shaft, is considerably greater in weight than the rotating elements of a conventional centrifugal pump of a similar pumping capacity. For example, the weight of the rotating elements of a conventional high-speed centrifugal pump may be twenty-five pounds while the rotating assembly of a comparably-sized pitot tube pump is four hundred pounds. The inertia in a pitot tube pump is likewise significantly greater than a high-speed centrifugal pump.
Systems with pitot tube pumps can be made to function in a manner which mimic high-speed centrifugal pumps in terms of their ability to rapidly change pump speed and, hence, pump capacity by adding valving systems which radically modify the flow of fluid from the pump within a very short period of time (e.g. four seconds). However, such modifications and means of rapidly increasing or decreasing the flow in a pitot tube pump are expensive.
A related problem inherent in pitot tube pumps is the fact that they require a greater amount of space, or they present a larger footprint, than high-speed centrifugal pumps. Pitot tube pumps are usually coupled with an induction motor and have a gear box positioned there between making the overall length of the pump, gear box and induction motor as much as ten feet long. By contrast, conventional high-speed centrifugal pumps powered by induction motors may be less than half the length of the pitot tube pump assemblage.
Another problem which is inherent in both pitot tube pump/induction motor assemblages and some high-speed centrifugal pump/induction motor assemblages is the need to align the drive shaft of the motor to the gear box, when used, and/or to the rotor or impeller shaft of the pump. The alignments are made by a technician in the field, which requires considerable time and expense to complete. Eliminating the need for alignment of the motor to a gear box and/or to the pump in an assemblage produces a significant operational cost advantage.
Further, while pitot tube pumps may be coupled directly to the housing of an induction motor and the rotor assembly coupled to the drive shaft, such arrangements present operating limitations. Principally, the drive shaft of an induction motor is small in diameter and, when coupled to a pitot tube rotor assembly, is required to support the large weight of the rotating assembly. As a result, the pump cannot be operated at high speeds, and low speeds must be maintained in order to avoid the occurrence of natural frequency vibration. Also, standard induction motors are not built with a bearing system that will accommodate the full range of thrust loading that a pitot tube pump applies to the drive shaft. Thus, a limitation is placed on the suction pressure that a pitot tube pump can handle and still have an adequate bearing life.
The problems identified above, which limit pitot tube pumps from being used in a number of pumping applications, may be solved by providing a pitot tube pump assembly where the pump is configured for connection to a motor that will provide the pitot tube pump assembly with the same or similar operational characteristics or advantages that are enjoyed by conventional centrifugal pumps coupled with standard induction motors, such as rapid modification of pump speed, a smaller footprint and/or significantly lowered operational costs.
In accordance with the present invention, a centrifugal pump, particularly of the pitot tube type, is configured for connection to a switched reluctance motor to produce a pump assembly that has improved pumping characteristics and operational advantages, thus rendering the inventive pump assembly suitable for use in applications where a conventional centrifugal pump and standard induction motor have previously been used. The improved characteristics and operational advantages of the present invention include the capability of providing rapid increase or decrease in the speed of the pump and providing a more compact footprint as compared to conventional pitot tube pump assemblages or centrifugal pump assemblages which employ a standard induction motor. The present invention also provides operational cost advantages over prior pump systems in being smaller and easier to install and operate, thereby rendering the inventive pitot tube pump more economical to manufacture and use.
The pump assembly of the present invention is particularly modified from the conventional configuration of a pitot tube pump to enable the direct attachment of the pump""s rotating assembly to the drive shaft of a switched reluctance motor. As described further hereinafter, the digital signal processing capabilities and configuration of switched reluctance motors make them particularly suitable for use with pitot tube pumps in view of the greater weight and higher inertia characteristics of pitot tube pumps, as compared with centrifugal pumps, particularly of the high-speed variety.
The rotating assembly of a pitot tube pump, comprising a rotor, rotor cover and shaft, is a considerably larger mass than the impeller of a high-speed centrifugal pump and often has a weight greater than 50 pounds (averaging 135 pounds or greater) and a rotating inertia greater than five pounds-foot2 (averaging 15 pounds-foot2 or greater). The mass and inertia of any given pump varies widely between types, sizes and manufactures of pumps. However, as a point of reference, a sixteen inch pitot tube pump may have a rotating mass of about 310 pounds and a rotating inertia of about 74 pounds-foot2. By comparison, the rotating assembly (i.e., impeller and shaft) of a typical high-speed centrifugal pump of the same pumping capacity has a rotating mass and inertia that are an order of magnitude less than the pitot tube pump. Therefore, to produce rapid changes in pump speed requires a suitable motor.
By xe2x80x9crapid changes in pump speedxe2x80x9d is meant that the pump speed capacity may be increased, for example, from zero percent capacity to eighty percent capacity or greater, or the pump speed capacity decreased, for example, from 100% to ten percent, or less, in a matter of seconds (i.e., between three to six seconds).
Standard induction motors and variable frequency drive motors are capable of producing rapid changes in pump speed in pumps having a smaller rotating mass and lower inertia. But the use of such motors with pitot tube pumps is economically unsuitable for producing quick changes in pump speed due to the greater mass and higher inertia of the rotating assembly.
Additionally, standard induction motors can be connected directly to the impeller of a conventional centrifugal pump to effect the desired rapid change in pump speed, but a similar direct attachment of the rotating assembly of a pitot tube pump to the drive shaft of a standard induction motor introduces certain limitations to operation of the pump. Standard induction motors are manufactured with a smaller drive shaft because the drive shaft is typically not intended to support a large mass at the end. Placing a large-mass rotating assembly of a pitot tube pump on the small drive shaft of an induction motor causes certain natural frequency vibrations that must be avoided. Therefore, the pump must be operated at lower speeds than may be necessary or desired.
Standard induction motors are also not built with a bearing system that is adequate to accommodate a full range of thrust loading that the rotating assembly applies to the drive shaft. The suction pressure that the pump can handle is therefore necessarily limited to maintain an adequate bearing life on the motor.
Using a switched reluctance motor in direct attachment to the rotating assembly of a pitot tube pump solves many of the aforementioned problems. The operational features of switched reluctance motors are known, but it is the ability of switched reluctance motors to electronically control the magnetic field, in concert with the ability to specifically configure switched reluctance motors to fit the operational requirements of the pump, which render a switched reluctance motor particularly suitable for use with pitot tube pumps to produce rapid changes in the pump speed.
The drive shaft dimensions of a switched reluctance motor can be modified to accommodate the particular mass of a pitot tube rotating assembly (i.e., the rotating assembly can vary significantly in size and mass), thereby allowing the drive shaft to be directly coupled to the rotating assembly and eliminating the operating limitations imposed by standard induction motors. The ability to accommodate the drive shaft of the switched reluctance motor to the pump""s mass requirements also eliminates suction pressure limitations and allows the bearings to be designed to the pump""s requirements.
The connection of the pump directly to the drive shaft of the motor eliminates the need for a gear box, which presents a significant cost savings, and thus eliminates the need for precision alignment of the pump shaft to the gear box shaft and to the drive shaft of the motor, which also presents another significant cost savings over conventional pitot tube arrangements. The ability to eliminate the gear box further allows the inventive pump assembly to have a significantly smaller footprint than conventional pitot tube pump/motor/gearbox assemblages. Elimination of the gear box also reduces the inertia of the system and enhances the rapid speed modification capability of the invention.
The speed of switched reluctance motors is controlled by changing the frequency of energizing and de-energizing the windings of the motor to create a rotating magnetic field. Thus, a switched reluctance motor may be programmed to rotate the drive shaft in either direction and to slow down, stop or start very quickly, thereby providing the rapid change in pump speed in a matter of seconds that is desired in some applications. Because of both mechanical and electrical design factors, switched reluctance motors can achieve greater speeds than a standard induction motor.
These advantages and others are more clearly realized from the following detailed description of the invention.